The two chief aims are (1) To continue to provide a detailed, well-illustrated account of the developing human nervous system based on histological examination of staged embryos of the Carnegie Collection (using the internationally accepted Carnegie staging system of Streeter and O'Rahilly). Silver impregnation of nerve fibers will be an essential feature, as will the preparation of precise graphic reconstructions, including the use of the Perspektomat for parallel perspective views. Tracts and nuclei of the developing brain, many of which appear earlier than has been appreciated previously, are being identified and mapped. Stages 11-23 are to be completed. (Stages 8 and 9 have been published by and stage 10 is being prepared.) (2) To incorporate appropriate data of stages 18-23 in computer-readable form at the Johns Hopkins Medical Instutions and analysed there, as has been done for stages 8-15 (in press) and is being undertaken for stages 16 and 17. This provides a previously unavailable precision in understanding the sequence and timing of developmental events, as well as the first appreciation of the (limited) degree of variation encountered at any one stage. The main long-term objective is to complete over a five-year period a precise, detailed study of the embryonic human nervous system at each developmental stage. (Animal studies are important but are not a substitute for the direct investigation of the human.) It is to be stressed that both the preparation of reconstructions and the selection, accumulation, and interpretation of the computer data re exceedingly time-consuming. The chief disciplines involved are human embryology and pathology (teratology), supplemented by computerized sorts and tabulations. The development of the nervous system is of fundamental biological and medical importance, and is crucial to an understanding of congenital anomalies.